Good morning…
I have experienced the same with one of mine. Sometimes it did not power down, sometimes otherwise around it did not power up. Resetting power fixed it and after restoring power to the Beolab 5 it worked fine again for 1 or 2 weeks.
Now since yesterday also this workaround does not function anymore. After restoring power to the Beolab 5 ist starts blinking red/green for a very short time changing to blinking red and then powers down. That´s it.
So that might be the same problem like with yours. Now I`m trying to maybe find somebody capable to do a repair on this one chassis. I might be able to manage removing the chassis, but not to do any repair on the electronis. So looking forward to find some help here and better to spend the money to somebody who is really deep into it as a Beo specialist 😍.